Of the challenges facing utilities, a major issue is the elimination of transmission constraints and bottlenecks. A significant issue in terms of grid utilization is active power flow control. Electric utility customers purchase real power, megawatts and MW-Hrs, as opposed to voltage or reactive power. Thus, control of how and where real power flows on the network is of critical importance. Congested networks limit system reliability and constrain the ability of low cost generators to provide interested customers with low-cost power. The situation is considerably aggravated when one sees that neighboring power lines are operating below capacity, but cannot be utilized, while uncontrolled ‘loop-flows’ result in overloads on existing lines. Active power flow control requires cost-effective ‘series VAR’ solutions that can alter the impedance of the power lines or change the angle of the voltage applied across the line, thus controlling power flow. Series reactive compensation has rarely been used other than on long transmission lines, mainly because of high costs and complexity of achieving voltage isolation and issues related to fault management.
There is general consensus that future power grids will need to be smart and aware, fault tolerant and self-healing, dynamically and statically controllable, and asset and energy efficient. The accepted and technically proven approach for realizing a smart grid, in particular achieving control of active power flow on the grid, has been through the use of Flexible AC Transmission Systems, or FACTS. Typical FACTS devices can operate at up to 345 kV and can be rated as high as 200 MVA. Even though FACTS technology is technically proven, it has not seen widespread commercial acceptance due to a number of reasons: 1) High system power ratings require the use of custom high power GTO or GCT devices with significant engineering effort—raises first cost; 2) High fault currents (60,000 Amps) and basic insulation requirements (1000 kV) stress the power electronic system, especially for series systems that are required for power flow control; 3) Utilities require higher reliability levels than what they have so far experienced with FACTS devices; 4) Required skilled work force in the field to maintain and operate the system is not within a utility's core competency normally; 5) High total cost of ownership, e.g., the Marcy convertible static compensator (CSC) cost $54 million.
The use of clamp-on transformers to realize ‘floating’ power couplers is well known. The technique has been proposed for coupling power from an insulated cable for underwater power transfer, and for contactless power transfer to mining equipment. The use of power line instrumentation that is floating on the power lines, and draws power from the line itself is also well known and has long been in commercial use. The use of floating couplers to realize power line communication, including broadband over power line (BPL) is also well known. The use of series coupled transformers to inject quadrature voltage into the line, as in a SSSC, UPFC or active filter is also well known.
Distributed series passive impedance use has been proposed by Hydro-Quebec, inserting switchable series capacitors on long transmission lines to change line impedance. The switches are generally controlled from a central controller. However, the line is specially built for desired impedance at significant cost and reduced flexibility. The desired impedance cannot be easily be attached to an existing line, and cannot be redeployed at a later date. Further, the capacitances can only decrease line impedance, and are primarily used to reduce the impedance of long-haul transmission lines.
The use of distributed series ‘active’ impedance modules has been proposed in U.S. patent application entitled “Distributed Floating Series Active Impedances For Power Transmission Systems,” having Ser. No. 10/678,966 and filed on Oct. 3, 2003, which is incorporated herein by reference in its entirety. The application proposes the use of power electronics inverters distributed along the line, to be used collectively to inject a quadrature voltage into the line to control current flow. The proposed technique requires a high bandwidth communications infrastructure that is used to command the impedance required from individual modules. The command is to be generated by a network level controller that has visibility to the current in all power lines, and can compute the optimal value for individual line impedances. This command is then communicated to individual modules for execution.
The complexity of the above-described mode of operation adds significant cost and complexity to the power transmission system. The cost of the power converters themselves, especially when designed to operate under the harsh environmental conditions encountered on a power transmission line, is likely to be a limiting factor. Further, the operation of power electronics converters for long periods of time (target 30 years) when suspended on a power transmission line and subject to harsh environmental conditions, will create reliability and availability problems for utilities deploying such technology. These issues point to the need for an alternative approach that has lower cost, is simpler, and is not predicated on the availability of a high bandwidth communications infrastructure.
There exists then a need for a distributed approach realizing passive devices, in particular series passive devices for distributed series impedance.